#include <bits/stdc++.h>
using namespace std;
int main(){
	int a,b,c,d;
	double s;
	cin>>d;
	a=d%10;
	b=d/10%10;
	c=d/100;
	s=(c+b)/(a+b)*1.0;
	cout<<s;
	return 0;
}
/**************************************************************
	Problem: 1608
	User: panyuchen
	Language: C
	Result: Compile Error
****************************************************************/